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Design and Application of the Reconstruction Software for the Babar Calorimeter Design and application of the reconstruction software for the BaBar calorimeter Philip David Strother Imperial College, November 1998 A thesis submitted for the degree of Doctor of Philosophy of the University of London, and Diploma of Imperial College 2 Abstract + The BaBar high energy physics experiment will be in operation at the PEP-II asymmetric e e− collider in Spring 1999. The primary purpose of the experiment is the investigation of CP violation in the neutral B meson system. The electromagnetic calorimeter forms a central part of the experiment and new techniques are employed in data acquisition and reconstruction software to maximise the capability of this device. The use of a matched digital filter in the feature extraction in the front end electronics is presented. The performance of the filter in the presence of the expected high levels of soft photon background from the machine is evaluated. The high luminosity of the PEP-II machine and the demands on the precision of the calorimeter require reliable software that allows for increased physics capability. BaBar has selected C++ as its primary programming language and object oriented analysis and design as its coding paradigm. The application of this technology to the reconstruction software for the calorimeter is presented. The design of the systems for clustering, cluster division, track matching, particle identification and global calibration is discussed with emphasis on the provisions in the design for increased physics capability as levels of understanding of the detector increase. The CP violating channel Bo J=ΨKo has been studied in the two lepton, two π0 final state. ! s The contribution of this channel to the evaluation of the angle sin 2β of the unitarity triangle is compared to that from the charged pion final state. An error of 0.34 on this quantity is expected after 1 year of running at design luminosity. Contents 1 CP violation in the B meson system 15 1.1 Introduction . 15 1.2 General phenomenology of mixing of neutral mesons . 15 1.3 CP violation and mixing in the K meson system . 17 1.3.1 Direct CP violation . 20 1.4 CP violation and mixing in the B meson system . 22 1.4.1 Phenomenology . 22 1.4.2 CP violation in B decays . 22 1.4.2.1 Direct CP violation . 22 1.4.2.2 CP violation in mixing . 23 1.4.2.3 CP violation in the interference between decays . 24 1.4.3 Measuring aξCP (t). 25 1.5 Quark mixing and the CKM matrix . 26 1.5.1 The CKM description of CP violation . 26 1.6 CKM Status and Current Constraints . 32 1.7 Chapter Summary . 33 2 The BaBar experiment and the PEP-II storage ring 35 2.1 Introduction . 35 2.2 The PEP-II storage ring . 35 2.2.1 Introduction . 35 2.2.2 The Main Storage Ring . 36 2.2.3 The Injection System . 38 4 2.2.4 The Interaction Region . 39 2.2.5 Machine backgrounds . 41 2.2.5.1 Synchrotron radiation background . 41 2.2.5.2 Lost beam particle backgrounds . 41 2.3 The BABAR experiment . 42 2.3.1 The Silicon Vertex Tracker . 44 2.3.1.1 Overview . 44 2.3.1.2 Detector Layout . 44 2.3.1.3 Electronics and readout . 46 2.3.2 The Drift Chamber . 46 2.3.2.1 Mechanical design . 46 2.3.2.2 Electronics . 48 2.3.3 The DIRC . 49 2.3.3.1 Mechanical construction . 49 2.3.3.2 Readout . 50 2.3.4 The Electromagnetic Calorimeter . 51 2.3.5 The Instrumented Flux Return . 52 2.3.5.1 RPC construction and readout . 54 2.3.6 Trigger . 55 2.4 Chapter Summary . 56 3 Software for the Electromagnetic Calorimeter 57 3.1 Introduction . 57 3.2 C++ and object oriented design . 57 3.2.1 The Concept of an Object . 57 3.2.2 Abstraction . 58 3.2.3 Encapsulation . 59 3.2.4 Object Oriented Design . 59 5 3.2.5 Relevance to physics capability . 59 3.2.6 The BABAR Framework Software . 60 3.3 Simulation software . 61 3.3.1 The calorimeter electronics simulation . 61 3.3.2 Digital filtering . 64 3.3.2.1 Theory of the Matched Filter . 64 3.3.2.2 Digital filter performance . 67 3.4 Reconstruction software . 68 3.4.1 Basic reconstruction . 68 3.4.1.1 Clustering . 68 3.4.1.2 Bump splitting . 69 3.4.2 Track–cluster matching . 70 3.4.3 Particle identification . 71 3.4.4 Offline calibration . 74 3.4.4.1 Retrieval . 75 3.4.4.2 Storage . 76 3.4.4.3 Offline calibration: summary . 77 3.5 Chapter summary . 77 4 Neutral particles in the electromagnetic calorimeter 79 4.1 Introduction . 79 4.2 Reconstruction parameters . 81 4.2.1 Clustering parameters . 81 4.2.1.1 Digi and seed thresholds . 82 4.2.1.2 Seed and cluster thresholds . 84 4.2.2 The CLEO clustering algorithm . 84 4.2.3 Cluster splitting parameters . 88 4.3 π0 and photon identification . 90 6 4.3.1 Discriminating variables . 90 4.4 Chapter Summary . 95 5 Study of the channel B0 J= K0 l+l π0π0 97 ! S ! − 5.1 Introduction . 97 5.2 Reconstructing the J= . 98 5.2.1 Tracking requirements . 98 5.2.2 J= reconstruction . 99 0 5.3 Reconstructing the KS . 100 5.4 Reconstructing the B0 . 104 5.5 Backgrounds . 105 0 5.5.1 Combinatorial backgrounds and J= KL . 105 5.5.2 Physics backgrounds . 106 5.5.3 Machine backgrounds . 107 5.6 CP reach . 108 5.6.1 Measuring ∆t . 108 5.6.2 Determination of the b quark flavour . 110 5.6.3 CP reach . 111 5.7 Chapter Summary . 111 6 Conclusions 113 A The Wigner–Weisskopf formalism 121 B Class diagrams 125 List of Figures 1-1 B meson decay rates as a function of lifetime for a CP asymmetry of Im(λ) = 0:75. If no CP violation were observed, the two decay rates would follow the exponential (solid line). 25 1-2 Unitarity triangles in the complex plane. Figure (a) depicts the triangle relating to possible CP violation in B physics. Figure (b) is the same triangle in the Wolfenstein parameterisation. 29 1-3 Figure (a) depicts a typical process for B0 mixing. Figure (b) is a typical decay, 0 B J= K0 . 30 ! S 1-4 Allowed regions in the ρ, η plane from current measurements. The dotted lines represent the boundaries of the allowed regions by allowing all parameters to vary. The contours, centred with a dot, represent the 95% confidence limits region using a given set of theoretical parameters. Limits from ∆mBs are not included. 33 2-1 Synchrotron radiation deposits from the low energy beam (top) and high energy beam (bottom). The Q1 quadrupole, being off axis w.r.t the low energy beam, and the B1 separation dipoles are the principle sources of synchrotron radiation at the IP. The darker shading indicates regions of higher photon density. 40 2-2 The BABAR detector. The 9.0 GeV electron beam travels left to right, the 3.1 GeV positron beam right to left. The interaction point is marked by the crosshairs. 43 2-3 The BABAR silicon vertex tracker. Cross sectional view in the r/φ plane. The beam pipe is shown in the centre. 43 2-4 The BABAR silicon vertex tracker. Cross sectional view in the z plane. The inter- action point is marked, indicating the asymmetry of PEP-II. The structures to the right and left are the beam separating fixed dipole magnets. 45 2-5 The BABAR drift chamber (z projection). The chamber is offset in z by 367mm to account for the asymmetry. The dimensions shown are in mm. 47 2-6 Section of the cell layout for the drift chamber. Axial layers are labeled A, stereo layers U and V. 48 2-7 The BABAR DIRC concept. Cerenkˇ ov radiation is internally reflected in the quartz bar, and emitted at the end of the bar preserving the Cerenkˇ ov angle. 50 8 2-8 Cross sectional view of the.
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